Some electronic devices utilize tunnel phenomenon. The tunnel phenomenon generally means a phenomenon that particles, e.g., electrons, etc., having lower kinetic energy than a potential barrier can pass the potential barrier to transit. The tunnel phenomenon is a phenomenon which cannot be explained by classical mechanics, but is characteristics of quantum mechanics and can be explained by quantum mechanics. A wave function of a particle propagates inside the potential barrier toward outside the potential barrier, attenuating, and unless an amplitude of the wave function is zero outside the potential barrier, the wave function propagates as a progressive wave, and can exit the potential barrier.
As examples of the tunnel phenomenon are known the phenomenon that α-particle is emitted from an atomic nucleus by α-decay, the phenomenon that electrons are emitted from the surface of a metal when a high voltage is applied to the metal (field emission), the phenomenon that when a high reverse bias is applied to a pn junction of a semiconductor, electrons punch through a depletion layer, and other phenomena. The tunnel phenomenon is practically a very important quantum mechanic effect.
A typical phenomenon as the tunnel phenomenon used in electronic devices is that when a voltage is applied to the metals on both sides of a “metal/insulator/metal” junction, a little current flows when the insulation is sufficiently thin. This phenomenon is one that takes place because an electron has a low probability of passing through an insulator, which usually does not conduct current, owing to the quantum mechanic effect when the insulator has a thickness of some angstroms (Å) to tens Å and is as thin as preferably some Å to ten-order Å. The current is called “tunnel current”, and a junction of such structure is called a “tunnel junction”.
In order to realize a very thin insulation layer for realizing the tunnel junction, usually an oxide film of a metal layer is used as an insulator barrier. For example, such insulator barrier is formed by oxidizing a surface layer of aluminum by suitable oxidation, such as natural oxidation, plasma oxidation, thermal oxidation or others. A film thickness of an oxide film can be controlled by adjusting oxidation conditions in accordance with used oxidation, and can have a required thickness of some angstroms (Å) to tens Å. The thus-formed aluminum oxide is a very thin insulator and can function as a barrier layer of a tunnel junction.
When a voltage is applied between the metals of the tunnel junction of the above-described “metal/oxide film /metal”, which are on both sides thereof, the current corresponding to the applied voltage characteristically exhibits non-linearity, which is different from linearity exhibited by the usual resistor. Electronic devices having such tunnel junction are used as non-linear devices.
Then, the structure “ferromagnetic metal/oxide film/ferromagnetic metal”, which has the metals of the “metal/oxide film/metal” on both sides replaced by the ferromagnetic metals is called “ferromagnetic tunnel junction”. It is known that in the ferromagnetic tunnel junction a tunnel probability (tunnel resistance) depends on a magnetized state of the magnetic layers on both sides. In other words, it is possible to control the tunnel resistance by changing a magnetized state of the magnetic layers on both sides by a magnetic field. When a relative angle between magnetic directions of both magnetic layers is represented by θ, a tunnel resistance R is expressed byR=Rs+0.5×ΔR(1−cos θ)  (1)
Rs represents a tunnel resistance at the time that a saturated magnetic field is applied. Two magnetic directions on both sides are oriented in directions of the magnetic field application. ΔR represents a change of the tunnel resistance.
What Formula (1) means is that when the two magnetic layers are magnetized in the same direction in a saturated magnetic filed, a relative angle θ=0° (cos θ=1), and a tunnel resistance R=Rs. In contrast to this, when the two magnetic layers are magnetized in direction opposite to each other in a saturated magnetic field, a relative angle in the magnetic directions θ=180° (cos θ=−1), and a tunnel resistance R=Rs+ΔR. In the absence of a magnetic field, as will be described later, one of the two magnetic layers has a magnetic direction fixed as the magnetic layer on the fixed side, and the other magnetic layer has a magnetic field direction weakly controlled in a domain as the magnetic layer on the free side so that the magnetic filed direction is orthogonal to a magnetic direction of the fixed side-magnetic layer. At this time, a relative angle between the magnetic direction of the two magnetic layers is θ=90° (cos θ=0), and a tunnel resistance R=Rs+0.5×ΔR.
That is, when magnetic directions of both magnetic layers agree with each other (θ=0°), a tunnel resistance R=Rs, which is minimum. When magnetic directions of both magnetic layers are opposite to each other (θ=180°), a tunnel resistance Ro=Rs+ΔR, which is maximum. Accordingly, magnetic directions of both magnetic layers are set in the absence of a magnetic field to be θ=90°, whereby a resistance value changes linearly, centering on a resistance value given when θ=90°, and linear outputs can be obtained.
Such phenomenon is attributable to that electrons in the ferromagnetic bodies are polarized. Usually electrons in a substance are up electrons, whose spin state is upward, and down electrons, whose spin state is downward. The non-magnetic metal has equal numbers of up electrons and down electrons, and does not exhibit magnetism as a whole non-magnetic metal. However, the magnetic metal has a number of up electrons (Nup) and a number of down electrons (Ndown) which are different from each other, and exhibits as a whole magnetic metal magnetism (i.e., up magnetism or down magnetism) of the electrons whose number is larger.
It is known that electrons tunnel one of the magnetic layers on both sides to the other magnetic layer through the thin oxide film, these electrons tunnel with spin states of the respective electrons retained. Accordingly, when an electron state of the tunneled magnetic layer has voids, the tunneling is possible, but the tunneling is impossible when the electron state of the tunneled magnetic layer has no void.
As expressed below, a tunnel resistance change ratio (ΔR/Rs) is expressed by using a product of a polarizability (also called deflected magnetic susceptibility) of a magnetic layer (to tunnel) as an electron source and a polarizability of a magnetic layer (to be tunneled).ΔR/Rs=2×P1×P2/(1−P1×P2)  (2)where P1 represents a polarizability of one of the magnetic layers, and P2 represents a polarizability of the other of the magnetic layers. A polarizability P of the magnetic layer is expressed as follows.P=2×(Nup−Ndown)/(Nup+Ndown)  (3)where Nup represents a number of up electron in the magnetic layer, and Ndown represents a number of down electrons in the magnetic layer.
A polarizability P of a magnetic layer depends on a kind of a ferromagnetic layer metal. However the magnetic layer often has a polarizability of approximately 50% depending on a kind, and in this case a tunnel resistance change ratio (ΔR/Rs) of tens percent can be expected.
As the conventionally known magnetoresistance (MR) effect, a resistance change ratio is about 0.6% for anisotropic magnetoresistance (AMR) effect, and for giant, magnetoresistance (GMR) effect, a resistance change ratio is some percentage to ten-order percent. The tunnel resistance change ratio is remarkably higher in comparison with the changes of AMR and GMR, and can be expected to be applied to magnetic heads, magnetic sensors, etc.
As a typical application of GMR to a magnetic head, the spin valve structure is known. The applicant of the present application has already proposed a TMR (tunnel-MR) head having the above-described ferromagnetic tunnel junction applied to the spin valve structure.
The spin valve structure uses a structure in which a magnetic metal layer is disposed between two magnetic layers, and an antiferromagnetic layer covers the upper surface of one of one of the magnetic layers so as to fix a magnetic direction of said one of the magnetic layers. As a ferromagnetic tunnel junction, a thin oxide film is disposed between two ferromagnetic layers as described above.
FIG. 1A is a sectional view explaining the ferromagnetic tunnel structure. The spin valve structure having the ferromagnetic tunnel junction typically comprises, as exemplified in FIG. 1A, a lower electrode 2 formed on a silicon substrate 1, a free-side magnetic layer 3 formed on the lower electrode, a first magnetical metal layer 4 formed on the free-side magnetic layer, an insulation layer 5 formed on the first magnetic metal layer, a second magnetic metal layer 6 formed on the insulation layer, a fixed-side magnetic layer 7 formed on the second magnetic metal layer, an antiferromagnetic layer 8 formed on the fixed-side magnetic layer, and an upper electrode 9 formed on the antiferromagnetic layer 8.
The lower electrode 2, the free-side magnetic layer 3 and the first magnetic metal layer 4 form a lower layer 10, and the second magnetic metal layer 6, the fixed-side magnetic layer 7, the antiferromagnetic layer 8 and the upper electrode 9 form an upper layer 12. A barrier layer 11 of an insulation layer 5 is formed between the lower layer 10 and the upper layer 12, isolating both from each other.
Respective members of the spin valve structure are as exemplified below. The substrate 1 is formed of silicon. The lower electrode 2 and the upper electrode 9 are respectively formed of a Ta (tantalum) film and has an about 50 nm-thickness. The free-side magnetic layer 3 and the fixed-side magnetic layer 7 are respectively formed of an NiFe film and has an about 17 nm-thickness. The first and the second magnetic metal layers 4, 6 are formed respectively of a Co (cobalt) film and has an about 3.3 nm-thickness. The insulation layer 5 is formed of an Al—Al2O3 film and has an about 1.3 nm-thickness. The antiferromagnetic layer 8 is formed of an FeMn film and has an about 45 nm-thickness.
The former NiFe film is one of two ferromagnetic layers and is called the free-side magnetic layer (free layer) 3 because its magnetic direction is not fixed. An Al—AlO film sandwiched between both Co films 4, 6 provides the barrier layer 11 formed of a thin aluminum oxide film, which forms the ferromagnetic tunnel junction. The second NiFe film is the other ferromagnetic layer and is called the fixed-side magnetic layer (pinned layer) 7 because a magnetic direction is fixed. The first magnetic metal layer 4 makes the same function as the free-side magnetic layer 3, and the second magnetic metal layer 6 makes the same function as the fixed-side magnetic layer 7. The FeMn film exchange-couples with the fixed-side magnetic layer 7 to fix a magnetic direction of the fixed-side magnetic layer and is called an antiferromagnetic layer (pinning layer) 8.
In the structure of such “free-side magnetic layer/insulation layer/fixed-side magnetic layer/antiferromagnetic layer”, when an external magnetic field (e.g., a signal magnetic field from a recording medium) is applied, magnetic directions of the free-side magnetic layer 3 and the first magnetic metal layer 4 alone are rotated. As a result, mainly a relative angle θ between a magnetic direction of the first magnetic metal layer 4 and that of the second magnetic metal layer 6 is changed, and a resistance change of the ferromagnetic tunnel junction is exhibited. That is, as shown by Formula (1), the tunnel resistance of the TMR (tunnel MR) changes depending on a magnetic field.
FIG. 1B is a schematic diagram explaining the measurement of resistance changes of a magnetic sensor using the ferromagnetic tunnel structure shown in FIG. 1A. A current source 39 is connected between the upper layer 12 and the lower layer 10, and certain current is charged. A voltage detector 40 is also connected between the upper layer 12 and the lower layer 10, and voltage changes between both layers are detected. When an external magnetic field (e.g., a signal magnetic field) is applied, a tunnel resistance of the ferromagnetic tunnel structure shown in FIG. 1A changes, and the tunnel resistance change is detected by the voltage detector 40 as a voltage change.
FIG. 2 shows a magnetoresistance effect curve of the tunnel structure using such spin valve structure. Based on FIG. 2, as an external magnetic field sequentially changes from −50 oersted (Oe) to −10 (Oe), to 0 (Oe), to +10 (Oe) and to +50 (Oe), reversible resistance change ratios of about 0.0% to about 0.0%, to about 10.0%, to about 20.0% and about 20.0% are exhibited. It has been found that the tunnel structure having the spin valve structure as shown in FIG. 2 exhibits substantially linear resistance change ratios of about 0% to 20% in an external magnetic field range of −10 (Oe) to +10 (Oe). Resistance change ratios of about 0% to 20% are exhibited in an external magnetic field range of −30 (Oe) to +30 (Oe). The resistance change ratios are converted to data of the logic [0], [1], whereby the resistance change ratios can be used in digital logic circuits.
However, in applying the tunnel structure having the spin valve structure to the magnetic sensor of a magnetic head, magnetic encoder or others, in a case that a device height h is too small, rotation of magnetic directions is often difficult near the edge of the device, which leads to a disadvantage that a sensitivity of the magnetic sensor is lowered.
In a case that practical device dimensions are in the order of some microns x some microns, when a device height h is decreased, static magnetic coupling of the fixed-side magnetic layer to the free-side magnetic layer becomes relatively stronger, and a magnetic direction of the free-side magnetic layer tends to be anti-parallel with a magnetic direction of the fixed-side magnetic layer, which makes it difficult for the magnetic direction to rotate in a direction of easy rotation. As a result, a sensitivity of the magnetoresistance effect device is lowered.
On the other hand, hard disk devices are prevalently used in electronic apparatuses because of their high speed of reading and writing data and large storage capacities.
The recent increase of storage capacities of the hard disk devices is remarkable, but further storage capacity increase is required.
To realize larger storage capacities of the hard disk devices it is an essential requirement that magnetic storage medium, i.e., magnetic disk mediums have higher recording densities. Recording density increase makes a recording bit of the magnetic recording medium smaller. It is necessary that the magnetic head is accordingly micronized, and the detection sensitivity is higher.
Recently, as a magnetic head of high detection sensitivity GMR (Giant Magneto-Resistance effect) head is proposed.
The GMR head is a magnetic head using the phenomenon that when an external magnetic field is applied to a layer film having a magnetic layer/a non-magnetic layer/a magnetic layer structure, an electric resistance of the layer film changes due to a difference between magnetized angles of the two magnetic layers, i.e., the GMR effect.
The GMR effect will be explained with reference to FIG. 3. FIG. 3 is a conceptual view of the GMR effect.
As shown in FIG. 3, the layer film 310 producing the GMR effect has the non-magnetic layer 316 sandwiched between the magnetic layer 314 and the magnetic layer 318. θ1 indicates a magnetized angle of the magnetic layer 314. θ2 indicates a magnetized angle of the magnetic layer 318. The magnetic layer 314 is magnetized at a magnetization vector M1, and the magnetic layer 318 is magnetized at a magnetization vector M2.
As shown in FIG. 3, a magnetic field is applied to the layer film 310 from the outside, a magnetized angle of the magnetic layer 314 becomes, e.g., θ1 and a magnetized angle of the magnetic layer 318 becomes, e.g., θ2.
When a difference between a magnetized angle θ1 and a magnetized angle θ2 is θ,θ=θ2−θ1
When an electric resistance at the time that no magnetic field is applied from the outside is Rs, an electric resistance R at the time that a magnetic field is applied from the outside is expressed byR=Rs+0.5×ΔR(1−cos Δθ)where ΔR is a constant which is different for materials of the layer film 310.
A value represented byΔR/Rs×100(%)is called an MR ratio. When the magnetic layer 314 is formed of, e.g., Co layer, the non-magnetic layer 316 is formed of, e.g., Cu layer, and the magnetic layer 318 is formed of, e.g., Co layer, the MR ratio is about 5-10%.
In using the layer film producing such GMR effect to a magnetic head, generally a structure called a spin valve is used. The spin valve structure is published in the specification of Japanese Patent Laid-Open Publication No. 358310/1992.
The spin valve structure will be explained with reference to FIG. 4. FIG. 4 is a sectional view showing a layer film of the spin valve structure.
As shown in FIG. 4, the layer film 410 of the spin valve structure is formed of a magnetic layer 414, a non-magnetic layer 416 and a magnetic layer 418, and an antiferromagnetic layer 420.
In the layer film of the three-layer structure simply formed of the magnetic layer 414, the non-magnetic layer 416 and the magnetic layer 418, a magnetic direction of the magnetic layer 414 and that of the magnetic layer 418 substantially agree with each other due to an external magnetic field, and a magnetized angle difference between a magnetized angle of the magnetic layer 414 and that of the magnetic layer 418 is very small.
Then, the layer film 410 of the spin valve structure has the antiferromagnetic layer 420 formed on the magnetic layer 418. The antiferromagnetic layer 420 fixes only a magnetic direction of a magnetic direction of the magnetic layer 418 contacting the antiferromagnetic layer 420. A magnetic direction of the magnetic layer 414 alone freely rotate corresponding to an external magnetic field. The magnetic layer 418, whose magnetic direction is fixed, is called a fixed layer, and the magnetic layer 414, whose magnetic direction freely rotates, is called a free layer.
A magnetic direction of the magnetic layer 418 is fixed constant, and a magnetic direction of the magnetic layer 414 is freely rotated by an external magnetic field, whereby an electric resistance R of the layer film 410 is changed corresponding to an external magnetic field.
Next, an operational principle of the magnetic head using the spin valve structure will be explained with reference to FIG. 5. FIG. 5 is a perspective view of the magnetic head using the spin valve structure, which shows the operational principle thereof.
As shown in FIG. 5, the layer film 410 of the spin valve structure formed of the free layer 414, the non-magnetic layer 416, the field layer 418 and the antiferromagnetic layer 420 is used as a core 400, and terminals 402 are formed on both ends of the core 400.
A magnetized angle θ1 of the free layer 414 is freely changed corresponding to a magnetic field 404 from a recording bit 332 of a magnetic storage medium, but an magnetized angle θ2 of the fixed layer 418 remains fixed. Thus, a difference between a magnetized angle θ1 of the free layer 414 and the magnetized angle θ2 of the fixed layer 48 can be made large, and electric resistance changes of the core 400 at the time that the recording bit 332 comes nearer can be larger.
However, as a magnetic record medium has higher recording density, a track width d1 is accordingly decreased. A core width d2 of the magnetic head must be decreased to correspond to a decreased track width d1. To this end, simply decreasing a core width d2 makes electric resistance changes of the core 400 smaller, which leads to lower detection sensitivity. Accordingly, when a core width d2 is decreased, a height h of the core 400 as well must be decreased.
However, when a height h of the core 400 is decreased, as shown in FIG. 6 a magnetic direction on the side of a signal detection plane 430 of the core 400 is not easily changed near the upper part of the core 400 under the influence of the demagnetizing field, and electric resistance changes of the core 400 are small. FIG. 6 indicates magnetic directions of the free layer 414 by the arrows when a height of the core 400 is, e.g., 5 μm. The region enclosed by the ellipse is a region where a magnetized angle θ1 becomes above a certain angle. As shown in FIG. 6, the region where a magnetized angle θ1 becomes above a certain angle, and additionally the magnetized angle θ1 is small.
Thus, the proposed magnetic head of the spin valve structure has the detection sensitivity much lowered when smaller-sized, and has found it difficult to adapt itself to higher density of the magnetic storage mediums.
In consideration of the above-described problems, a first object of the present invention is to provide a novel magnetic sensor, a magnetic head, and an encoder.
A second object of the present invention is to provide a magnetic sensor, a magnetic head and an encoder which have the tunnel junction, ensure rotation of magnetic directions of the free-side magnetic layer, and have good sensitivity.
A third object of the present invention is to provide a magnetic head which is adaptable to higher density of magnetic record mediums, and a hard disk device of a large storage capacity using the magnetic head.